266 AGRICULTURAL ANALYSIS 



amounted to 1.6 gram and the calcium phosphate in this quan- 

 tity to have been determined as 1.540 gram, the calcium fluorid is 

 thus proved to be 0.060 gram, and, therefore, 2:0.060: :ioo:x= 3 

 per cent, calcium fluorid, which, multiplied by 0.4897, gives 1.46 

 per cent, of fluorin. 



The above method, while shorter, is not to be preferred to the 

 former process when great accuracy is desired. All the solu- 

 ble silica may not fall out of the solution as Wyatt says. Again, 

 the method of separating the phosphoric acid can not be regarded 

 as strictly accurate. Finally, the fluorin is calculated from small 

 differences in the weight of very heavy precipitates, and all the 

 error of the process may be found affecting the numbers for 

 fluorin. For commercial purposes, however, the method has the 

 merit of comparative brevity. 



228. Method of Rose. Clarke and Hillebrand recommend 

 the Rose modification of the method just described, in which 

 chromium and any residue of phosphoric acid are removed by 

 silver nitrate.* 5 The previous separation of silica and alumina by 

 carbonate of ammonium is advised instead of nitrate or chlorid, to 

 avoid loss of fluorin on evaporation. By whatever method the 

 silica is thrown out, the alkaline carbonate must be converted 

 into nitrate and not chlorid if chromium and phosphorus are 

 present. The solution at the time the silver nitrate is added 

 should contain enough of undecomposed alkaline carbonate to 

 cause a copious precipitation of silver carbonate in order to take 

 up the acid set free. After heating and filtering the excess of 

 silver is to be removed by sodium or potassium chlorid, sodium 

 carbonate added, and the fluorin precipitated by calcium chlorid 

 in excess. No ammonium salts should be present in the solution 

 when the calcium chlorid is added, for these tend to hold the 

 fluorin in solution. The remaining part of the operation is con- 

 ducted as above described. Attention is called to the fact that 

 there is no very satisfactory qualitative test for the presence of 

 fluorin. The usual method of heating the powdered substance by 

 the blowpipe, with sodium metaphosphate on platinum foil, is 

 not always reliable. 



74 U. S. Geological Survey, Bulletin 148, 1897 : 57. 



